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Editorial improvements to backup and warm-standby documentation.

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REL8_2_STABLE
Tom Lane 19 years ago
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  1. 451
      doc/src/sgml/backup.sgml

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doc/src/sgml/backup.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.94 2006/11/10 22:32:20 tgl Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.95 2006/12/01 03:29:15 tgl Exp $ -->
<chapter id="backup">
<title>Backup and Restore</title>
@ -18,7 +18,7 @@
<itemizedlist>
<listitem><para><acronym>SQL</> dump</para></listitem>
<listitem><para>File system level backup</para></listitem>
<listitem><para>Continuous Archiving</para></listitem>
<listitem><para>Continuous archiving</para></listitem>
</itemizedlist>
Each has its own strengths and weaknesses.
</para>
@ -180,12 +180,14 @@ pg_dump -h <replaceable>host1</> <replaceable>dbname</> | psql -h <replaceable>h
<title>Using <application>pg_dumpall</></title>
<para>
The above mechanism is cumbersome and inappropriate when backing
up an entire database cluster. For this reason the <xref
linkend="app-pg-dumpall"> program is provided.
<application>pg_dump</> dumps only a single database at a time,
and it does not dump information about roles or tablespaces
(because those are cluster-wide rather than per-database).
To support convenient dumping of the entire contents of a database
cluster, the <xref linkend="app-pg-dumpall"> program is provided.
<application>pg_dumpall</> backs up each database in a given
cluster, and also preserves cluster-wide data such as users and
groups. The basic usage of this command is:
cluster, and also preserves cluster-wide data such as role and
tablespace definitions. The basic usage of this command is:
<synopsis>
pg_dumpall &gt; <replaceable>outfile</>
</synopsis>
@ -197,7 +199,9 @@ psql -f <replaceable class="parameter">infile</replaceable> postgres
but if you are reloading in an empty cluster then <literal>postgres</>
should generally be used.) It is always necessary to have
database superuser access when restoring a <application>pg_dumpall</>
dump, as that is required to restore the user and group information.
dump, as that is required to restore the role and tablespace information.
If you use tablespaces, be careful that the tablespace paths in the
dump are appropriate for the new installation.
</para>
</sect2>
@ -210,7 +214,7 @@ psql -f <replaceable class="parameter">infile</replaceable> postgres
to dump such a table to a file, since the resulting file will likely
be larger than the maximum size allowed by your system. Since
<application>pg_dump</> can write to the standard output, you can
just use standard Unix tools to work around this possible problem.
use standard Unix tools to work around this possible problem.
</para>
<formalpara>
@ -284,7 +288,7 @@ pg_dump -Fc <replaceable class="parameter">dbname</replaceable> &gt; <replaceabl
</sect1>
<sect1 id="backup-file">
<title>File system level backup</title>
<title>File System Level Backup</title>
<para>
An alternative backup strategy is to directly copy the files that
@ -450,7 +454,7 @@ tar -cf backup.tar /usr/local/pgsql/data
<para>
If we continuously feed the series of WAL files to another
machine that has been loaded with the same base backup file, we
have a <quote>hot standby</> system: at any point we can bring up
have a <firstterm>warm standby</> system: at any point we can bring up
the second machine and it will have a nearly-current copy of the
database.
</para>
@ -502,7 +506,7 @@ tar -cf backup.tar /usr/local/pgsql/data
available hardware, there could be many different ways of <quote>saving
the data somewhere</>: we could copy the segment files to an NFS-mounted
directory on another machine, write them onto a tape drive (ensuring that
you have a way of restoring the file with its original file name), or batch
you have a way of identifying the original name of each file), or batch
them together and burn them onto CDs, or something else entirely. To
provide the database administrator with as much flexibility as possible,
<productname>PostgreSQL</> tries not to make any assumptions about how
@ -605,7 +609,7 @@ archive_command = 'test ! -f .../%f &amp;&amp; cp %p .../%f'
<para>
Note that although WAL archiving will allow you to restore any
modifications made to the data in your <productname>PostgreSQL</> database
modifications made to the data in your <productname>PostgreSQL</> database,
it will not restore changes made to configuration files (that is,
<filename>postgresql.conf</>, <filename>pg_hba.conf</> and
<filename>pg_ident.conf</>), since those are edited manually rather
@ -685,10 +689,10 @@ SELECT pg_start_backup('label');
<programlisting>
SELECT pg_stop_backup();
</programlisting>
This should return successfully; however, the backup is not yet fully
valid. An automatic switch to the next WAL segment occurs, so all
WAL segment files that relate to the backup will now be marked ready for
archiving.
This terminates the backup mode and performs an automatic switch to
the next WAL segment. The reason for the switch is to arrange that
the last WAL segment file written during the backup interval is
immediately ready to archive.
</para>
</listitem>
<listitem>
@ -700,7 +704,7 @@ SELECT pg_stop_backup();
already configured <varname>archive_command</>. In many cases, this
happens fairly quickly, but you are advised to monitor your archival
system to ensure this has taken place so that you can be certain you
have a valid backup.
have a complete backup.
</para>
</listitem>
</orderedlist>
@ -727,15 +731,13 @@ SELECT pg_stop_backup();
It is not necessary to be very concerned about the amount of time elapsed
between <function>pg_start_backup</> and the start of the actual backup,
nor between the end of the backup and <function>pg_stop_backup</>; a
few minutes' delay won't hurt anything. However, if you normally run the
few minutes' delay won't hurt anything. (However, if you normally run the
server with <varname>full_page_writes</> disabled, you may notice a drop
in performance between <function>pg_start_backup</> and
<function>pg_stop_backup</>. You must ensure that these backup operations
are carried out in sequence without any possible overlap, or you will
invalidate the backup.
</para>
<para>
<function>pg_stop_backup</>, since <varname>full_page_writes</> is
effectively forced on during backup mode.) You must ensure that these
steps are carried out in sequence without any possible
overlap, or you will invalidate the backup.
</para>
<para>
@ -758,7 +760,7 @@ SELECT pg_stop_backup();
</para>
<para>
To make use of this backup, you will need to keep around all the WAL
To make use of the backup, you will need to keep around all the WAL
segment files generated during and after the file system backup.
To aid you in doing this, the <function>pg_stop_backup</> function
creates a <firstterm>backup history file</> that is immediately
@ -855,7 +857,7 @@ SELECT pg_stop_backup();
Restore the database files from your backup dump. Be careful that they
are restored with the right ownership (the database system user, not
root!) and with the right permissions. If you are using tablespaces,
you may want to verify that the symbolic links in <filename>pg_tblspc/</>
you should verify that the symbolic links in <filename>pg_tblspc/</>
were correctly restored.
</para>
</listitem>
@ -975,15 +977,17 @@ restore_command = 'cp /mnt/server/archivedir/%f %p'
<para>
If recovery finds a corruption in the WAL data then recovery will
complete at that point and the server will not start. The recovery
process could be re-run from the beginning, specifying a
<quote>recovery target</> so that recovery can complete normally.
complete at that point and the server will not start. In such a case the
recovery process could be re-run from the beginning, specifying a
<quote>recovery target</> before the point of corruption so that recovery
can complete normally.
If recovery fails for an external reason, such as a system crash or
the WAL archive has become inaccessible, then the recovery can be
simply restarted and it will restart almost from where it failed.
Restartable recovery works by writing a restart-point record to the control
file at the first safely usable checkpoint record found after
<varname>checkpoint_timeout</> seconds.
if the WAL archive has become inaccessible, then the recovery can simply
be restarted and it will restart almost from where it failed.
Recovery restart works much like checkpointing in normal operation:
the server periodically forces all its state to disk, and then updates
the <filename>pg_control</> file to indicate that the already-processed
WAL data need not be scanned again.
</para>
@ -1173,48 +1177,6 @@ restore_command = 'copy /mnt/server/archivedir/%f "%p"' # Windows
</para>
</sect2>
<sect2 id="backup-incremental-updated">
<title>Incrementally Updated Backups</title>
<indexterm zone="backup">
<primary>incrementally updated backups</primary>
</indexterm>
<indexterm zone="backup">
<primary>change accumulation</primary>
</indexterm>
<para>
Restartable Recovery can also be utilised to offload the expense of
taking periodic base backups from a main server, by instead backing
up a Standby server's files. This concept is also generally known as
incrementally updated backups, log change accumulation or more simply,
change accumulation.
</para>
<para>
If we take a backup of the server files whilst a recovery is in progress,
we will be able to restart the recovery from the last restart point.
That backup now has many of the changes from previous WAL archive files,
so this version is now an updated version of the original base backup.
If we need to recover, it will be faster to recover from the
incrementally updated backup than from the base backup.
</para>
<para>
To make use of this capability you will need to setup a Standby database
on a second system, as described in <xref linkend="warm-standby">. By
taking a backup of the Standby server while it is running you will
have produced an incrementally updated backup. Once this configuration
has been implemented you will no longer need to produce regular base
backups of the Primary server: all base backups can be performed on the
Standby server. If you wish to do this, it is not a requirement that you
also implement the failover features of a Warm Standby configuration,
though you may find it desirable to do both.
</para>
</sect2>
<sect2 id="continuous-archiving-caveats">
<title>Caveats</title>
@ -1287,23 +1249,23 @@ restore_command = 'copy /mnt/server/archivedir/%f "%p"' # Windows
<title>Warm Standby Servers for High Availability</title>
<indexterm zone="backup">
<primary>Warm Standby</primary>
<primary>warm standby</primary>
</indexterm>
<indexterm zone="backup">
<primary>PITR Standby</primary>
<primary>PITR standby</primary>
</indexterm>
<indexterm zone="backup">
<primary>Standby Server</primary>
<primary>standby server</primary>
</indexterm>
<indexterm zone="backup">
<primary>Log Shipping</primary>
<primary>log shipping</primary>
</indexterm>
<indexterm zone="backup">
<primary>Witness Server</primary>
<primary>witness server</primary>
</indexterm>
<indexterm zone="backup">
@ -1311,132 +1273,131 @@ restore_command = 'copy /mnt/server/archivedir/%f "%p"' # Windows
</indexterm>
<indexterm zone="backup">
<primary>High Availability</primary>
<primary>high availability</primary>
</indexterm>
<para>
Continuous Archiving can be used to create a High Availability (HA)
cluster configuration with one or more Standby Servers ready to take
over operations in the case that the Primary Server fails. This
capability is more widely known as Warm Standby Log Shipping.
Continuous archiving can be used to create a <firstterm>high
availability</> (HA) cluster configuration with one or more
<firstterm>standby servers</> ready to take
over operations if the primary server fails. This
capability is widely referred to as <firstterm>warm standby</>
or <firstterm>log shipping</>.
</para>
<para>
The Primary and Standby Server work together to provide this capability,
though the servers are only loosely coupled. The Primary Server operates
in Continuous Archiving mode, while the Standby Server operates in a
continuous Recovery mode, reading the WAL files from the Primary. No
The primary and standby server work together to provide this capability,
though the servers are only loosely coupled. The primary server operates
in continuous archiving mode, while each standby server operates in
continuous recovery mode, reading the WAL files from the primary. No
changes to the database tables are required to enable this capability,
so it offers a low administration overhead in comparison with other
replication approaches. This configuration also has a very low
performance impact on the Primary server.
so it offers low administration overhead in comparison with some other
replication approaches. This configuration also has relatively low
performance impact on the primary server.
</para>
<para>
Directly moving WAL or "log" records from one database server to another
is typically described as Log Shipping. <productname>PostgreSQL</>
implements file-based log shipping, which means that WAL records are batched one file at a time. WAL
is typically described as log shipping. <productname>PostgreSQL</>
implements file-based log shipping, which means that WAL records are
transferred one file (WAL segment) at a time. WAL
files can be shipped easily and cheaply over any distance, whether it be
to an adjacent system, another system on the same site or another system
on the far side of the globe. The bandwidth required for this technique
varies according to the transaction rate of the Primary Server.
Record-based Log Shipping is also possible with custom-developed
procedures, discussed in a later section. Future developments are likely
to include options for synchronous and/or integrated record-based log
shipping.
varies according to the transaction rate of the primary server.
Record-based log shipping is also possible with custom-developed
procedures, as discussed in <xref linkend="warm-standby-record">.
</para>
<para>
It should be noted that the log shipping is asynchronous, i.e. the
WAL records are shipped after transaction commit. As a result there
can be a small window of data loss, should the Primary Server
suffer a catastrophic failure. The window of data loss is minimised
by the use of the <varname>archive_timeout</varname> parameter,
which can be set as low as a few seconds if required. A very low
setting can increase the bandwidth requirements for file shipping.
is a window for data loss should the primary server
suffer a catastrophic failure: transactions not yet shipped will be lost.
The length of the window of data loss
can be limited by use of the <varname>archive_timeout</varname> parameter,
which can be set as low as a few seconds if required. However such low
settings will substantially increase the bandwidth requirements for file
shipping. If you need a window of less than a minute or so, it's probably
better to look into record-based log shipping.
</para>
<para>
The Standby server is not available for access, since it is continually
The standby server is not available for access, since it is continually
performing recovery processing. Recovery performance is sufficiently
good that the Standby will typically be only minutes away from full
good that the standby will typically be only moments away from full
availability once it has been activated. As a result, we refer to this
capability as a Warm Standby configuration that offers High
Availability. Restoring a server from an archived base backup and
rollforward can take considerably longer and so that technique only
really offers a solution for Disaster Recovery, not HA.
</para>
<para>
When running a Standby Server, backups can be performed on the Standby
rather than the Primary, thereby offloading the expense of
taking periodic base backups. (See
<xref linkend="backup-incremental-updated">)
</para>
<para>
Other mechanisms for High Availability replication are available, both
commercially and as open-source software.
</para>
<para>
In general, log shipping between servers running different release
levels will not be possible. It is the policy of the PostgreSQL Global
Development Group not to make changes to disk formats during minor release
upgrades, so it is likely that running different minor release levels
on Primary and Standby servers will work successfully. However, no
formal support for that is offered and you are advised not to allow this
to occur over long periods.
capability as a warm standby configuration that offers high
availability. Restoring a server from an archived base backup and
rollforward will take considerably longer, so that technique only
really offers a solution for disaster recovery, not HA.
</para>
<sect2 id="warm-standby-planning">
<title>Planning</title>
<para>
On the Standby server all tablespaces and paths will refer to similarly
named mount points, so it is important to create the Primary and Standby
servers so that they are as similar as possible, at least from the
perspective of the database server. Furthermore, any <xref
linkend="sql-createtablespace" endterm="sql-createtablespace-title">
commands will be passed across as-is, so any new mount points must be
created on both servers before they are used on the Primary. Hardware
need not be the same, but experience shows that maintaining two
identical systems is easier than maintaining two dissimilar ones over
the whole lifetime of the application and system.
</para>
<para>
There is no special mode required to enable a Standby server. The
operations that occur on both Primary and Standby servers are entirely
normal continuous archiving and recovery tasks. The primary point of
It is usually wise to create the primary and standby servers
so that they are as similar as possible, at least from the
perspective of the database server. In particular, the path names
associated with tablespaces will be passed across as-is, so both
primary and standby servers must have the same mount paths for
tablespaces if that feature is used. Keep in mind that if
<xref linkend="sql-createtablespace" endterm="sql-createtablespace-title">
is executed on the primary, any new mount point needed for it must
be created on both the primary and all standby servers before the command
is executed. Hardware need not be exactly the same, but experience shows
that maintaining two identical systems is easier than maintaining two
dissimilar ones over the lifetime of the application and system.
In any case the hardware architecture must be the same &mdash; shipping
from, say, a 32-bit to a 64-bit system will not work.
</para>
<para>
In general, log shipping between servers running different major release
levels will not be possible. It is the policy of the PostgreSQL Global
Development Group not to make changes to disk formats during minor release
upgrades, so it is likely that running different minor release levels
on primary and standby servers will work successfully. However, no
formal support for that is offered and you are advised to keep primary
and standby servers at the same release level as much as possible.
When updating to a new minor release, the safest policy is to update
the standby servers first &mdash; a new minor release is more likely
to be able to read WAL files from a previous minor release than vice
versa.
</para>
<para>
There is no special mode required to enable a standby server. The
operations that occur on both primary and standby servers are entirely
normal continuous archiving and recovery tasks. The only point of
contact between the two database servers is the archive of WAL files
that both share: Primary writing to the archive, Standby reading from
that both share: primary writing to the archive, standby reading from
the archive. Care must be taken to ensure that WAL archives for separate
servers do not become mixed together or confused.
primary servers do not become mixed together or confused.
</para>
<para>
The magic that makes the two loosely coupled servers work together
is simply a <varname>restore_command</> that waits for the next
WAL file to be archived from the Primary. The <varname>restore_command</>
is specified in the <filename>recovery.conf</> file on the Standby
Server. Normal recovery processing would request a file from the
WAL archive, causing an error if the file was unavailable. For
Standby processing it is normal for the next file to be
is simply a <varname>restore_command</> used on the standby that waits for
the next WAL file to become available from the primary. The
<varname>restore_command</> is specified in the <filename>recovery.conf</>
file on the standby
server. Normal recovery processing would request a file from the
WAL archive, reporting failure if the file was unavailable. For
standby processing it is normal for the next file to be
unavailable, so we must be patient and wait for it to appear. A
waiting <varname>restore_command</> can be written as a custom
script that loops after polling for the existence of the next WAL
file. There must also be some way to trigger failover, which
should interrupt the <varname>restore_command</>, break the loop
and return a file not found error to the Standby Server. This then
ends recovery and the Standby will then come up as a normal
and return a file-not-found error to the standby server. This
ends recovery and the standby will then come up as a normal
server.
</para>
<para>
Sample code for the C version of the <varname>restore_command</>
would be:
Pseudocode for a suitable <varname>restore_command</> is:
<programlisting>
triggered = false;
while (!NextWALFileReady() && !triggered)
@ -1452,14 +1413,14 @@ if (!triggered)
<para>
<productname>PostgreSQL</productname> does not provide the system
software required to identify a failure on the Primary and notify
the Standby system and then the Standby database server. Many such
tools exist and are well integrated with other aspects of a system
failover, such as IP address migration.
software required to identify a failure on the primary and notify
the standby system and then the standby database server. Many such
tools exist and are well integrated with other aspects required for
successful failover, such as IP address migration.
</para>
<para>
Triggering failover is an important part of planning and
The means for triggering failover is an important part of planning and
design. The <varname>restore_command</> is executed in full once
for each WAL file. The process running the <varname>restore_command</>
is therefore created and dies for each file, so there is no daemon
@ -1467,8 +1428,8 @@ if (!triggered)
handler. A more permanent notification is required to trigger the
failover. It is possible to use a simple timeout facility,
especially if used in conjunction with a known
<varname>archive_timeout</> setting on the Primary. This is
somewhat error prone since a network or busy Primary server might
<varname>archive_timeout</> setting on the primary. This is
somewhat error prone since a network problem or busy primary server might
be sufficient to initiate failover. A notification mechanism such
as the explicit creation of a trigger file is less error prone, if
this can be arranged.
@ -1479,54 +1440,55 @@ if (!triggered)
<title>Implementation</title>
<para>
The short procedure for configuring a Standby Server is as follows. For
The short procedure for configuring a standby server is as follows. For
full details of each step, refer to previous sections as noted.
<orderedlist>
<listitem>
<para>
Setup Primary and Standby systems as near identically as
Set up primary and standby systems as near identically as
possible, including two identical copies of
<productname>PostgreSQL</> at the same release level.
</para>
</listitem>
<listitem>
<para>
Setup Continuous Archiving from the Primary to a WAL archive located
in a directory on the Standby Server. Ensure that both <xref
Set up continuous archiving from the primary to a WAL archive located
in a directory on the standby server. Ensure that <xref
linkend="guc-archive-command"> and <xref linkend="guc-archive-timeout">
are set. (See <xref linkend="backup-archiving-wal">)
are set appropriately on the primary
(see <xref linkend="backup-archiving-wal">).
</para>
</listitem>
<listitem>
<para>
Make a Base Backup of the Primary Server. (See <xref
linkend="backup-base-backup">)
Make a base backup of the primary server (see <xref
linkend="backup-base-backup">), and load this data onto the standby.
</para>
</listitem>
<listitem>
<para>
Begin recovery on the Standby Server from the local WAL
Begin recovery on the standby server from the local WAL
archive, using a <filename>recovery.conf</> that specifies a
<varname>restore_command</> that waits as described
previously. (See <xref linkend="backup-pitr-recovery">)
previously (see <xref linkend="backup-pitr-recovery">).
</para>
</listitem>
</orderedlist>
</para>
<para>
Recovery treats the WAL Archive as read-only, so once a WAL file has
been copied to the Standby system it can be copied to tape at the same
time as it is being used by the Standby database server to recover.
Thus, running a Standby Server for High Availability can be performed at
the same time as files are stored for longer term Disaster Recovery
Recovery treats the WAL archive as read-only, so once a WAL file has
been copied to the standby system it can be copied to tape at the same
time as it is being read by the standby database server.
Thus, running a standby server for high availability can be performed at
the same time as files are stored for longer term disaster recovery
purposes.
</para>
<para>
For testing purposes, it is possible to run both Primary and Standby
For testing purposes, it is possible to run both primary and standby
servers on the same system. This does not provide any worthwhile
improvement on server robustness, nor would it be described as HA.
improvement in server robustness, nor would it be described as HA.
</para>
</sect2>
@ -1534,78 +1496,127 @@ if (!triggered)
<title>Failover</title>
<para>
If the Primary Server fails then the Standby Server should begin
If the primary server fails then the standby server should begin
failover procedures.
</para>
<para>
If the Standby Server fails then no failover need take place. If the
Standby Server can be restarted, even some time later, then the recovery
If the standby server fails then no failover need take place. If the
standby server can be restarted, even some time later, then the recovery
process can also be immediately restarted, taking advantage of
Restartable Recovery. If the Standby Server cannot be restarted, then a
full new Standby Server should be created.
restartable recovery. If the standby server cannot be restarted, then a
full new standby server should be created.
</para>
<para>
If the Primary Server fails and then immediately restarts, you must have
a mechanism for informing it that it is no longer the Primary. This is
If the primary server fails and then immediately restarts, you must have
a mechanism for informing it that it is no longer the primary. This is
sometimes known as STONITH (Shoot the Other Node In The Head), which is
necessary to avoid situations where both systems think they are the
Primary, which can lead to confusion and ultimately data loss.
primary, which can lead to confusion and ultimately data loss.
</para>
<para>
Many failover systems use just two systems, the Primary and the Standby,
Many failover systems use just two systems, the primary and the standby,
connected by some kind of heartbeat mechanism to continually verify the
connectivity between the two and the viability of the Primary. It is
also possible to use a third system, known as a Witness Server to avoid
connectivity between the two and the viability of the primary. It is
also possible to use a third system (called a witness server) to avoid
some problems of inappropriate failover, but the additional complexity
may not be worthwhile unless it is set-up with sufficient care and
rigorous testing.
</para>
<para>
At the instant that failover takes place to the Standby, we have only a
Once failover to the standby occurs, we have only a
single server in operation. This is known as a degenerate state.
The former Standby is now the Primary, but the former Primary is down
and may stay down. We must now fully recreate a Standby server,
either on the former Primary system when it comes up, or on a third,
possibly new, system. Once complete the Primary and Standby can be
The former standby is now the primary, but the former primary is down
and may stay down. To return to normal operation we must
fully recreate a standby server,
either on the former primary system when it comes up, or on a third,
possibly new, system. Once complete the primary and standby can be
considered to have switched roles. Some people choose to use a third
server to provide additional protection across the failover interval,
server to provide backup to the new primary until the new standby
server is recreated,
though clearly this complicates the system configuration and
operational processes (and this can also act as a Witness Server).
operational processes.
</para>
<para>
So, switching from Primary to Standby Server can be fast but requires
So, switching from primary to standby server can be fast but requires
some time to re-prepare the failover cluster. Regular switching from
Primary to Standby is encouraged, since it allows the regular downtime
that each system requires to maintain HA. This also acts as a test of the
failover mechanism so that it definitely works when you really need it.
primary to standby is encouraged, since it allows regular downtime on
each system for maintenance. This also acts as a test of the
failover mechanism to ensure that it will really work when you need it.
Written administration procedures are advised.
</para>
</sect2>
<sect2 id="warm-standby-record">
<title>Implementing Record-based Log Shipping</title>
<title>Record-based Log Shipping</title>
<para>
The main features for Log Shipping in this release are based
around the file-based Log Shipping described above. It is also
possible to implement record-based Log Shipping using the
<function>pg_xlogfile_name_offset()</function> function (see <xref
linkend="functions-admin">), though this requires custom
development.
<productname>PostgreSQL</productname> directly supports file-based
log shipping as described above. It is also possible to implement
record-based log shipping, though this requires custom development.
</para>
<para>
An external program can call <function>pg_xlogfile_name_offset()</>
An external program can call the <function>pg_xlogfile_name_offset()</>
function (see <xref linkend="functions-admin">)
to find out the file name and the exact byte offset within it of
the latest WAL pointer. If the external program regularly polls
the server it can find out how far forward the pointer has
moved. It can then access the WAL file directly and copy those
bytes across to a less up-to-date copy on a Standby Server.
the current end of WAL. It can then access the WAL file directly
and copy the data from the last known end of WAL through the current end
over to the standby server(s). With this approach, the window for data
loss is the polling cycle time of the copying program, which can be very
small, but there is no wasted bandwidth from forcing partially-used
segment files to be archived. Note that the standby servers'
<varname>restore_command</> scripts still deal in whole WAL files,
so the incrementally copied data is not ordinarily made available to
the standby servers. It is of use only when the primary dies &mdash;
then the last partial WAL file is fed to the standby before allowing
it to come up. So correct implementation of this process requires
cooperation of the <varname>restore_command</> script with the data
copying program.
</para>
</sect2>
<sect2 id="backup-incremental-updated">
<title>Incrementally Updated Backups</title>
<indexterm zone="backup">
<primary>incrementally updated backups</primary>
</indexterm>
<indexterm zone="backup">
<primary>change accumulation</primary>
</indexterm>
<para>
In a warm standby configuration, it is possible to offload the expense of
taking periodic base backups from the primary server; instead base backups
can be made by backing
up a standby server's files. This concept is generally known as
incrementally updated backups, log change accumulation or more simply,
change accumulation.
</para>
<para>
If we take a backup of the standby server's files while it is following
logs shipped from the primary, we will be able to reload that data and
restart the standby's recovery process from the last restart point.
We no longer need to keep WAL files from before the restart point.
If we need to recover, it will be faster to recover from the incrementally
updated backup than from the original base backup.
</para>
<para>
Since the standby server is not <quote>live</>, it is not possible to
use <function>pg_start_backup()</> and <function>pg_stop_backup()</>
to manage the backup process; it will be up to you to determine how
far back you need to keep WAL segment files to have a recoverable
backup. You can do this by running <application>pg_controldata</>
on the standby server to inspect the control file and determine the
current checkpoint WAL location.
</para>
</sect2>
</sect1>

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